Filtration of Aluminium Melts using Ceramic Foam Filters (CFF) and Electromagnetic Field
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AbstractThe objective of the experimental trials was to investigate the influence of the electromagnetic force generated by an AC powered solenoid on ceramic foam filters (CFF) of different filter grades (30, 50 and 80 ppi). The filtering and priming of liquid aluminium alloy (A356) with the addition of SiC particles based A356 alloy was investigated. A batch reactor was used to investigate the priming and wetting properties of the CFF´s. The filtration mechanisms and the ability to remove gas trapped inside and below the filter elements with and without the application of magnetic fields were studied. Each filter grade was studied with three different times, i.e. 3, 6 and 10 minutes. Gravity trials (without electromagnetic field) were performed for each filter grade as a standard reference to industry practice. Industrial standard CFF´s (Sivex®) were combined with an electromagnetic field generated by short double coils (solenoids) using a frequency of 50 Hz. The induction heating power input was sufficient to maintain the temperature of the melt. The magnetic field penetrated the entire filter element and generated a force (Lorentz) in the melt, resulting in zones of differential pressure. The pressure gradient caused high velocity recirculation of the melt. Lorentz force calculations were performed analytically and the solutions are verified by COMSOL® simulations and by experimental observations. The base alloy (A356) was mixed with 10 % composite material of A356 base alloy, containing 15 % SiC-particles, sized from 13 to 23 µm. Experiments with 30 ppi grade filters were performed with a higher amount of particles using a recipe of: 60 % A356, 20 % anodized and lacquered plates and 20% composite containing 15 % SiC particles sized from 13 to 23 µm. The results of these experimental trials show the application of magnetic fields during the filtration process, affecting collection mechanisms and the distribution of SiC particles inside the filter media. Significant improvements of the priming and wetting of the liquid aluminium alloy on the alumina were observed with the application of a magnetic field. Gas bubbles trapped inside and below the filter element were completely removed. Electromagnetic forces were observed to influence the separation of the particles in addition to all of the standard filtration mechanisms. Preheating was not necessary when using electromagnetically enhanced priming of the filter media. The induced velocity strongly increases the throughput of the melt through the finer filter grades.